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Full text of "Text Book Of Mechanical Engineering"

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902                            Appendix //.

free hydrochloric acid, thus causing corrosion; but it may be
neutralised with lime. If the preceding precautions be not
adopted, viz., applying the right chemicals as indicated by feed-
water analysis, a good deal of chipping must be done to remove
scale and prevent overheating. (See Appendix V., p. 1006.)

P. 6pj. Boiler Covering.—To reduce radiation from
boilers or pipes, pasty coverings are usually applied, which are
more or less effective. The following results were obtained by
Mr. Laird, of St. Louis, who made analyses and heat tests of the
materials indicated:—

WEIGHT COMPOSITION PER CENT.


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	'55
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Plastic magnesia ...
	
	
	7
	
	
	
	
	334

Sectional n-agnesia
	
	Q2'2
	7-8
	
	
	
	
	•37r-?

Asbestos fire -felt ...
	
	82
	
	iS
	
	
	
	367*5

Asbestos   sponge,
	
	
	
	
	
	
	
	

moulded  ......
	oc-8
	
	4*2
	
	
	
	
	171*^

Fossil meal ......
	
	
	
	12
	80
	8
	
	^76*2

Plaster of Paris and
	
	
	
	
	
	
	
	

sawdust    ......
	?
	
	
	?
	
	...
	
	438

Asbestos  fire -felt
	
	
	
	
	
	
	
	

cement
	3'5
	
	32
	
	
	
	64-5
	5637

Asbestos    sponge
	
	
	
	
	
	
	
	

cement     ......
	10
	
	31
	
	
	
	59
	604

Bare pipe    ......
	
	
	
	
	
	
	
	1085


	
	
	
	j    —
	
	
	
	

jP. 693. Boiler Fittings.'—Safety valve area may vary
considerably: thus, the Board of Trade (p. 694) require only
two-thirds of the area used in America. The former, based on
experiments up to 60 Ibs. pressure, should be smaller at high
pressures, but larger again for forced draught The lift is always
small, varying from '2" at 10 Ibs. to '02" at 30 Ibs. blow off, but is
proportionately larger with a small than a large valve.

Steam Traps are for automatically removing condensed water
in steam pipes, in one of two ways: either the rise of water lifts